In the semiconductor industry, step-and-repeat alignment and exposure systems are employed in the processing of semiconductive wafers to form integrated circuits. Very large scale integrated circuits are often fabricated by utilizing a precisely controlled stage to successively position adjacent regions containing an integral number of individual microcircuits on a semiconductive wafer with respect to an image (formed by a projection lens of the step-and-repeat alignment and exposure system) of a reticle containing a next level of microcircuitry that is then printed on the semiconductive wafer at each of those regions. This step-and-repeat printing operation forms an array of adjacent regions of microcircuitry on the semiconductive wafer in rows and columns in an ordered parallel and orthogonal manner. Successive processing of the semiconductive wafer and printing of a further level of microcircuitry, aligned with the preceding processed regions to a high (sub micron) accuracy, are typically employed in the fabrication of integrated circuits from the semiconductive wafer.
Two problems that are critical to all step-and-repeat alignment and exposure systems are the manner in which they receive their alignment signals and the manner in which they perform alignments. Some known prior art systems utilize a tv camera tube to receive an enlarged re-image of an alignment mark, as illuminated by an image of an alignment window, and computer-analyze a time history of signals coming from scan lines of the tube. The computer then commands alignment servos to move a stage of such a step-and-repeat alignment and exposure system a calculated distance along a calculated direction vector in order to reduce alignment error to a predicted minimum. Generally, the system is then required to verify that the alignment has been made satisfactorily. A more detailed description of such tv scanning systems is provided as background material in companion U.S. patent application Ser. No. 692,011, filed Jan. 14, 1985, now U.S. Pat. No. 4,585,337, and entitled IMPROVED STEP-AND-REPEAT ALIGNMENT AND EXPOSURE SYSTEM by Edward H. Phillips and incorporated by reference herein.
Such systems work well, but are slowed by the scan and computation time which requires a serial decoupling of the alignment system from the alignment servos. An example of a system that presents an analog alignment signal is described in detail in the afore mentioned U.S. Pat. No. 4,391,494, incorporated by reference herein. That system utilizes a single detection element (a photomultiplier tube) to sample light intensity reflected from a similarly illuminated alignment mark. Since there is no vectorial information presented by that system, mechanical motion is required to generate alignment information and the resulting iterative alignment process is relatively slow in execution.
What is needed is an alignment system, and utilization method therefore, which presents both amplitude and vectorial information and can operate, together with the alignment servos, in real time.
Another problem faced by many step-and-repeat alignment and exposure systems is the limited acquisition range of their alignment mark systems. Such a limited acquisition range can make it difficult to acquire an alignment signal from a newly loaded wafer. This is because the newly loaded wafer is positioned mechanically on a wafer chuck of the step-and-repeat alignment and exposure system with limited accuracy. The resulting wafer alignment mark location can be outside of the area illuminated by an image of an alignment window of the system with the result that no information is present in its alignment signal.
What is required is a new alignment window and mark system, and utilization method therefore, that characteristically features enlarged acquisition range and preserves the real time operation called for hereinbefore.
In order to facilitate the adaptation of the improved alignment method, and alignment window and mark systems, to a unit magnification catadioptric lens, it would be highly desirable to utilize an improved viewing port and a viewing microscope, to allow direct splitfield microscope viewing of the image of the reticle on the surface of the semiconductive wafer. Such features are not taught by the afore mentioned and incorporated U.S. Pat. No. 4,391,494. The combination of a suitable viewing port and an optimal splitfield microscope for this purpose is taught in the afore mentioned and incorporated U.S. patent application Ser. No. 692,011. Alternately, the splitfield microscope of U.S. patent application Ser. No. 692,011 can be used with yet another improved viewing port presented hereinafter.
Accordingly, it is the principal object of this invention to provide a reticle alignment window array and wafer alignment mark system, and utilization method therefore, which presents both amplitude and vectorial information and can operate, together with alignment servos of a step-and-repeat alignment and exposure system, in real time.
Another object of this invention is to provide the reticle alignment window array and wafer alignment mark system, and utilization method therefore, with features that enlarge its acquisition range and preserve the real time operation called for hereinbefore.
Another object of this invention is to provide an improved viewing port which presents a darkfield image of an image field of a unit magnification catadioptric lens of the step-and-repeat alignment and exposure system for microscope viewing.
Another object of this invention is to provide a viewing port which presents a brightfield image of the image field of the unit magnification catadioptric lens of the step-and-repeat alignment and exposure system for microscope viewing.
Another object of this invention is to provide a microscope system able to present enlarged re-images of selected portions of the image of the image field of the unit magnification catadioptric lens of the step-and-repeat alignment and exposure system present at the viewing port; which allows measurement of the intensity of the light reflected from illuminated portions of the alignment mark by sub-systems adapted for measuring light intensity.
Another object of this invention is to provide sub-systems adapted for measuring the intensity of the light reflected from the illuminated portions of each alignment mark.
Another object of this invention is to provide an electronic system adapted for coupling the outputs of the sub-systems adapted for measuring the intensity of the light reflected from the illuminated portions of each alignment mark into the alignment servos of a stage of the step-and-repeat alignment and exposure system in a manner suitable for aligning the wafer alignment mark within the image of the reticle alignment window array in real time.
Another object of this invention is to provide the stage of the step-and-repeat alignment and exposure system with a stage reference sub-system for providing an alignment reference for an image of the reticle alignment window array by presenting a stage reference mark image when illuminated by the image of the reticle.
Another object of this invention is to provide the step-and-repeat alignment and exposure system with a sub-system for translating and rotating an orthogonal axes of motion of a main stage to achieve compatibility with the actual position and orientation of the image of the reticle.
Another object of this invention is to provide the step-and-repeat alignment and exposure system with a sub-system for globally aligning the semiconductive wafer and shooting "blind".
Another object of this invention is to provide the step-and-repeat alignment and exposure system with a sub-system for aligning each previously processed region of the semiconductive wafer to the image of the reticle prior to photometrically printing the image of the reticle on the region.
Another object of this invention is to provide a method of utilizing the apparatus of the invention to calibrate the apparatus.
Another object of this invention is to provide a method of utilizing the calibrated apparatus of the invention to photometrically print first level semiconductive wafers.
Still another object of this invention is to provide a method of utilizing the calibrated apparatus of the invention to photometrically print higher level semiconductive wafers.
These and other objects, which will become apparent from an inspection of the accompanying drawings and a reading of the associated description, are accomplished by the present invention comprising a main stage controlled for movement in a plane defined by first and second orthogonal axes; a wafer chuck for supporting the semiconductive wafer wherein said wafer chuck is supported on the main stage for rotational positioning about a third axis orthogonal to the first and second orthogonal axes; catadioptric projection lens means for imaging portions of a reticle onto the semiconductive wafer or onto a reference mark associated with the main stage, wherein an optical path is defined through the reticle and catadioptric lens means; a light source for supplying illumination or exposure light; additional imaging lens means supplementing the catadioptric projection lens means and positioned along the optical path for viewing a projected conjugate image of the portions of the semiconductive wafer or reference mark which are illuminated by the projected image of the reticle; means for viewing selected portions of the projected conjugate image; and means for utilizing the viewed selected portions of the projected conjugate image.
More specifically, the above are accomplished according to the illustrated preferred embodiments of this invention by providing an improved step-and-repeat alignment and exposure system and method therefore including a main stage controlled for movement to different positions along orthogonal X and Y axes; a wafer chuck mounted on the main stage and adapted for rotational movement about a third orthogonal Z axis for supporting a semiconductive wafer thereon; an optical subassembly mounted on the main stage for imaging a stage reference mark into the plane of the upper surface, or circuit side, of the semiconductive wafer; a projection lens of the unit magnification catadioptric type for imaging illuminated portions of a reticle onto portions of the semiconductive wafer of the image of the stage reference mark, depending on the position to which the main stage is moved; a light source for directing uniform illumination of exposure light along an optical path extending thru the reticle and the projection lens; a viewing port created by an imaging lens whose aperture is filled by light passing thru a main mirror of the projection lens, either thru a physical aperture hole or a beam splitting coating of the main mirror; so that, at the viewing port, a projected conjugate image may be viewed, wherein the image includes the selected portions of the semiconductive wafer or the image of the stage reference mark, illuminated by the projected image of the illuminated portions of the reticle, in darkfield or brightfield respectively; a pair of novelly constructed, infinity corrected microscope objectives adapted for viewing selected portions of the projected conjugate image; wherein the selected portions are coupled, thru a novel use of the infinity correction principal, to a pair of focusing lenses for re-imaging the viewed, selected portions of the projected conjugate image upon a pair of light sensitive, diode arrays.
The improved step-and-repeat alignment and exposure system and method therefore also includes an alignment sub-system and method able to provide a real time, multidimensional, offset signal representative of the distance and direction required to move the stage to achieve a selected alignment of the image of a reticle alignment window array with either a stage reference mark image or a pair of wafer alignment marks on the semiconductive wafer, so as to minimize stage alignment time.
Also included are a new reticle alignment window array and stage reference or wafer alignment marks with which to implement the alignment sub-system and method, and, able to provide real time alignment and enlarged acquisition range compatible with normal mechanical positioning of the semiconductive wafer on the wafer chuck.
Another sub-system and method are provided which rotate and translate the X,Y co-ordinate axes of motion of the stage into offset and rotated U,V co-ordinate axes of motion of the stage for achieving compatibility with the actual position of the reticle.
A further sub-system and method are provided which provide global alignments and subsequent "blind" shooting of the semiconductive wafer for minimizing wafer processing time.
Finally, a sub-system and method are included which provide regional alignments and immediate exposure of adjacent regions of a semiconductive wafer for minimizing alignment errors.